Module 9: Blood Vessels
Blood vessels: it is through these that the blood is pumped and distributed to all areas of the
Anatomy – General Organization
The circulatory system is essentially a closed system of tubes (blood vessels) filled with fluid
(blood) that is moved around by a central pump (the heart).
The blood vessels consist of :
o Arteries and arterioles that transport the blood away from the heart
o Capillaries where gas exchange takes place
o Venules and veins that return the blood back to the heart.
The large arteries branch into smaller arteries, which eventually turn into smaller arterioles.
These arterioles also branch into smaller vessels that lead to the capillaries.
Capillaries are the smallest of all the blood vessels and are the functional units of the circulatory
system where substances enter and leave.
The capillaries converge into small venules, which get larger and larger to form veins
There are two principal loops that the blood takes through the body.
One loop begins on the right side of the heart and sends blood through arteries to the lungs.
o These blood vessels continually branch into smaller and smaller blood vessels, which
eventually become capillaries.
o Gas exchange takes place in these pulmonary capillaries
o Oxygen diffuses into the blood and carbon dioxide out. The blood then enters venules
and progressively larger veins to eventually return to the left side of the heart.
This loop is called the pulmonary circulation. The second loop begins on the left side of the heart.
The freshly oxygenated blood is now pumped to the rest of the body; it travels from the left
ventricle, through the aorta, and into arteries.
o The arteries branch into smaller arterioles that, in turn, branch into capillaries. Again,
the capillaries are the site of gas exchange.
o Oxygen, nutrients, hormones, etc. are delivered to the cells, and carbon dioxide (CO2)
and waste products are picked up. This deoxygenated blood returns to the right side of
the heart through venules and larger veins.
This circulatory loop is called the systemic circulation.
There are two smaller circulatory loops within the larger systemic circulation:
o The hepatic portal loop found in the digestive system
o The hypothalamic-hypophyseal portal system found in the brain
Blood Volume distribution
The total blood volume (TBV) of an average human being is roughly 5 litres (1.3 gallons)
Largest portion (70%) is contained in the veins
o Since they contain the most “capacity,” the veins are referred to as the capacitance
vessels or blood “reservoir”
Heart and lungs contain about 15%
The arteries contain about 10% of TBV
Capillaries (where gas exchange occurs) contain last 5%
Blood velocity and Cross-sectional area of vessels
Just like blood volume, blood pressure, blood velocity, and cross-sectional areas of the blood
vessels also vary throughout the circulation. These characteristics have important functional
significance as you can see below.
o Have highest blood pressure and velocity,
o A very low cross-sectional area o As a consequence, these vessels rapidly distribute the blood throughout the body.
o Have a lower blood pressure and velocity
o The cross-sectional area is higher
o These vessels are the site of highest resistance in the circulation and help regulate blood
flow to an organ
o The blood velocity is lowest
o Their total cross-sectional area is the highest in the circulation.
o These characteristics help to maximize exchange of substances across these blood
The blood pressure and cross-sectional area decrease while blood velocity increases in the
venules and veins. These vessels return the blood back to the heart while also storing a large
percentage of the total blood volume
Pressure, Flow, and Resistance
You will remember from module 3 that the driving force moving ions during diffusion is a
Here, the force that moves the blood through the entire circulatory system is a pressure
The diagram at above shows a large drop in pressure from high (in the aorta) to low (in the
veins). This is the pressure gradient that causes the blood to flow through both the pulmonary
and systemic circulation. The higher the pressure gradient, the higher the blood flow
The largest drop in blood pressure throughout the systemic circulation occurs in the arterioles
Blood flows through vessels as a result of the pressure gradient. As it flows, it encounters
This resistance is the result of blood "dragging" along the walls of the vessels.
o The higher the resistance that the blood encounters, the lower the flow
o As a result, blood flow through a vessel is called laminar (streamlined) flow: there
are thin "layers" of flow whose velocity varies across the vessel—flow is slower at
the edges and faster in the center
o Therefore, in order to examine blood flow through a vessel, we have to consider the
pressure gradient and the resistance the fluid encounters.
We can calculate blood flow (F) using equation 1
(P1 – P2) is the pressure gradient. This is simply the pressure change between two points (P1
and P2). R is the resistance.
Resistance to blood flow
As mentioned earlier, resistance to flow comes from the blood "dragging" along the walls of the vessels.
Therefore, several factors can affect the resistance:
1. The thickness or viscosity of the fluid
The thicker the fluid, the higher the resistance.
Generally, the viscosity of the blood does not change.
2. The length of the vessel
The longer the blood vessel, the higher the resistance.
Since the vessels in the body are of constant length and do not change over short
periods of time, the length of the vessel is not a major factor.
3. The most important factor is the diameter (or radius) of the blood vessel.
The smaller the inside diameter, the higher the resistance. In order to calculate the resistance in the blood vessel, we must take into account the
viscosity of the fluid and the length and radius of the blood vessel as shown in equation 2.
Since the viscosity of the blood does not usually change and the length of the vessels
generally remains constant over short periods of time. Therefore, we can simplify equation
2 by removing viscosity and length, resulting in equation 3.
Simply put, equation 3 states that if the radius of a blood vessel decreases, the resistance
increases and vice versa (that is, they are inversely proportional).
If we take equation 3 and combine it with equation 1 from earlier, the result is equation 4.
According to equation 4, if you alter either the pressure gradient or the radius of the vessel,
you will alter blood flow
ANIMATION: You should notice that a small change in the radius results in a large change in
blood flow. To produce an equal change in blood flow using only the pressure gradient would
require a very large change in pressure!
Control of Blood Flow in the Body
There are two ways the body can alter blood flow-by changing either the pressure gradient
or the radius of the vessel
Since blood pressure is generally kept relatively constant, the best way to regulate blood
flow through an organ is by changing the radius of the vessels supplying it.
o Decreasing the radius of the vessel feeding an organ, the resistance in that vessel will
o This increase in resistance will decrease the flow of blood into that organ.
As illustrated at right, arterioles are generally the vessels that control blood flow in an organ.
Note that as the blood flow decreases in the lower arteriole, the blood flow in the other
three arterioles increases to maintain a constant flow of 5 L/min. Changing blood flow in response to needs of an organ
Changing the diameter of the blood vessel that supplies an organ can alter the blood flow to it.
Why would you want to do this?
Blood flow to an organ will depend on the needs of that organ for oxygen or nutrients.
For example, after a meal, blood flow is diverted away from muscle to the intestine to help with
the digestion of food. Conversely, when exercising, blood is diverted away from the intestine to
the working muscle to supply it with oxygen and nutrients while removing carbon dioxide.
Diverting the blood is achieved by altering the radius of the arterioles—by either:
o Vasodilating them (making them wider) or
o Vasoconstricting them (making them narrower)
o Recall that changing the diameter will either increase or decrease blood flow.
Blood pressure and resistance throughout the systemic circulation
When doctors measure blood pressure, they listen for specific tapping sounds called Korotkoff's
The blood produces these sounds when flow becomes turbulent as it "squeezes" through blood
vessels pinched off by the pressure cuff as the pressure is released. o The pressure when the sound first appears represents the systolic pressure.
o The pressure when the sound disappears (when flow becomes laminar again) is the
The pressure in the aorta and the large arteries is pulsatile.
In a normal, healthy individual, it fluctuates between a systolic pressure of 120 mmHg (when
the heart contracts) and a diastolic pressure of 80 mmHg (when the heart relaxes).
Since the aorta and large arteries are very elastic and have a large radius, there is very little
resistance to the blood. As a result, the pressure remains high in these vessels
The fall in pressure begins in the small arteries where the resistance to the blood begins to
The greatest drop in pressure occurs in the arterioles due to the very large resistance (the
largest in the entire systemic circulation).
o The pressure decreases from roughly 80 mmHg to about 30 mmHg.
o The pressure continues to drop in the capillary from 30 to 10 mmHg
o Then from 10 to 5 mmHg in the veins.
o A small but significant amount of resistance is present in the venous circulation;
thus, by the time blood reaches the right atrium, the pressure is almost 0 mmHg
QUESTION: highest resistance to blood flow is measured in the arterioles or capillaries?
The main purpose of the cardiovascular system is to circulate blood, thereby delivering
oxygen and nutrients to the cells while removing carbon dioxide and waste
In order to reach all of these cells, blood must flow through a series of ever-branching vessels
that get smaller and smaller and then reconverge and get bigger.
The driving force behind this flow of blood is a pressure gradient, which is high at the
beginning of the circulatory system and low at the end. As the blood flows through the
vessels, it encounters resistance that can slow the flow of blood.
The flow of blood can be redirected to organs based on their needs for oxygen and nutrients
by a combination of vasoconstricting and vasodilating arterioles.
Structure of Blood Vessels
The main purpose of the cardiovascular system is to deliver the oxygen- and nutrient-rich blood
to the cells of the body and to remove carbon dioxide and waste.
This section will examine how the cardiovascular system performs this function. In order to do
so, however, we must look at the structure of the blood vessels—particularly that of the
In many ways, these structural characteristics are responsible for the differences in pressure,
resistance, and volume that can be found throughout the circulatory system Arteries and veins contain three layers in their walls.
o The outermost layer, called the tunica externa, is composed mostly of fibrous
o The middle layer, or tunica media, consists of smooth muscle and elastic tissue.
o The innermost layer, called the tunica interna, is composed of endothelial cells.
Along with these three layers, veins also contain valves to ensure blood flows in one direction—
back to the heart.
Capillaries are composed entirely of a single layer of endothelial cells
o These thin walls permit the diffusion of substances into and out of the blood.
Arteries have walls that contain a larger proportion of elastic tissue. These vessels must be
able to withstand and absorb the large pulsatile pressure changes during contractions of the
Vein walls are thinner than arteries. They contain some smooth muscle and a little elastic
tissue. This makes them more flexible and distensible; therefore, they are able to contain
70% of the total blood volume. o The small amount of muscle tissue and the presence of valves allow these vessels to
constrict, propelling blood back to the heart
Arterioles contain mostly smooth muscle and are able to constrict or dilate to redirect blood
to and from organs
Venules contain no smooth muscle or elastic tissue since blood pressure is very low and their
function is essentially to return blood to the veins. Contain mainly fibrous tissue.
Capillaries are composed entirely of endothelial cells, facilitating the diffusion of substances
into and out of the blood
Examine the relative amounts of fibrous, muscle, elastic, and endothelial tissue frm figures
below Exchange of Substances across the capillary
Capillaries are the main sites for the exchange of oxygen, nutrients, water, carbon dioxide,
The movement of these substances is enhanced by the very thin endothelial cell and also the
presence of clefts and fenestrations in the capillary.
o These holes allow the movement of water and most dissolved solutes (except large
proteins) into and out of the blood.
The movement of dissolved substances across the capillary occurs by:
The Capillary – Diffusion
Diffusion is the random movement of a solute down its concentration gradient
Both oxygen and carbon dioxide are lipid soluble so they can diffuse right through the capillary
o Oxygen and nutrients are in high concentration in the blood and they diffuse into
o Carbon dioxide and waste products diffuse into the blood as their concen